Soldered modules that include substrates of differing substrate materials are subject to thermal stress due to differing coefficients of thermal expansion (CTE) between the respective substrates. For example, due to CTE, cracks in the solder and possibly elsewhere (the substrates themselves) occur during the reflow process when attempting to solder a ceramic substrate to a printed circuit board (PCB) material such as FR4.
Differing CTEs among substrates is also commonly known as CTE mismatch. A typical PCB material can have a CTE of 15 PPM/Degree C. while a Ceramic substrate can have a CTE of 6 PPM/Degree C. Thus, cracks are initiated during the cooling process (of the reflow process) as the solder materials between the PCB and ceramic solidifies while the PCB and ceramic are not fully in a steady state. Note that Pb-free solder paste (95.5% Sn-3.8% Ag-0.7% Cu) solidifies at 217 degree C. while the PCB and the Ceramic are still contracting at different rates as the temperature is reduced. As a result, cracks form in the ceramic and possibly the solder as well.